Engineering Phase to Reinforce Dielectric Polarization in Nickel Sulfide Heterostructure for Electromagnetic Wave Absorption.

Engineering phase transition in micro-nanomaterials to optimize the dielectric properties and further enhance the electromagnetic microwave absorption (EMA) performance is highly desirable. However, the severe synthesis conditions restrict the design of EMA materials featuring controllable phases, which hinders the tunability of effective absorption bandwidth (EAB) and leads to an unclear loss mechanism. Herein, a seed phase decomposition-controlled strategy is proposed to induct nickel sulfide (NiSx ) absorbers with controllable phases and hollow sphere nature. Transmission electron microscopy holography and theoretical calculations evidence that the reconstruction of atoms in phase transition induces numerous heterogeneous interfaces and lattice defects/sulfur vacancies to cause varied work functions and local electronic redistribution, which contributes to reinforced dielectric polarization. As a result, the optimized NiS2 /NiS heterostructure enables enhanced EM attenuation capability with a wide EAB of 5.04 GHz at only 1.6 mm, compared to that of NiS2 and NiS. Moreover, the correlation between EAB and NiS phase content is demonstrated as the "volcano" feature. This study on the concept of phase transition of micro-nanomaterials can offer a novel approach to constructing highly efficient absorbers for EMA and other functionalities.

Phase Transition Induced via the Template Enabling Cocoon-like MoS2 an Exceptionally Electromagnetic Absorber.

Structural engineering via the template method is efficient for micro-nano assembling. However, only structural design and lack of composition control restrict their advanced application. To overcome this issue, applying a template to simultaneously realize the structural design and fine component control is highly desired, which has been ignored. In this study, a spinel-shaped MoS2 heterostructure with controlled phase ratios of 1H and 2H phase is developed using the AlOOH template method. This work demonstrates that the MoS2 phase transition mechanism from 2H to 1T is substantially attributed to the close exposed crystal's surface and approximately accordant surface energy. The superiority and additional proof are provided based on density-functional theory simulation, transmission electron microscope holography, etc. With an effective absorptance region of 6.3 GHz under a thickness of 1.4 mm, the reported samples present outstanding microwave absorption capacity. This is attributed to the beneficial coupled effect between the well-designed structure and phase regulation. This work offers valuable insights into structural engineering and component regulation template methods.

Enhanced electrocatalytic hydrogen evolution by molybdenum disulfide nanodots anchored on MXene under alkaline conditions.

Efficient hydrogen production through electrocatalysis represents a promising path for the future clean energy. Molybdenum disulfide (MoS2) is a good substitute for platinum-based catalysts, due to its low cost and high activity. However, the limited active sites and low electrical conductivity of MoS2 hinder its large-scale industrial application under alkaline conditions. Herein, we constructed MoS2 nanodots anchored on an MXene/nickel foam (MoS2 NDs/MXene/NF) heterostructure by a cascade polymerization synthesis and in situ vulcanization. The prepared heterostructure displays an ultralow overpotential of 94 mV at a current density of 10 mA cm-2 with a Tafel slope of only 59 mV dec-1 in alkaline (1 M KOH) hydrogen evolution reaction (HER), and is better than conventional MoS2 electrocatalysts reported so far. Fine structural analysis indicates that MoS2 NDs are dispersed uniformly on the surface of the heterostructure with consistent orientation, leading to the improvement of MoS2 conductivity with more paths for electron transfer. Moreover, the orientation of the synthesized MoS2 NDs was verified to expose the more (002) crystal plane, which exhibits higher activity than other planes. Our results demonstrate that MoS2 NDs with heterostructure design and preferential growth can serve as high-efficiency noble-metal free electrocatalysts for the HER in alkaline solution.

Morphology-Evolved Succulent-like FeCo Microarchitectures with Magnetic Configuration Regulation for Enhanced Microwave Absorption.

The regulation of magnetic configuration through diverse morphologies to achieve a rapid magnetic response has attracted considerable academic favor on account of the unique application prospects in various fields. Herein, porous FeCo alloys with morphology evolved from spheres to succulent-like microstructures are successfully constructed via a facile hydrothermal reaction-hydrogen reduction synthetic strategy. A multiple balance/competition mechanism is proposed, including the coexistence of the dissolution-precipitation balance of hydroxides and the dissociation-stability balance of coordination compounds, the Fe3+-Co2+ competition, and the precipitation-coordination reaction contest. As the morphology evolves to a succulent-like assembly, the multidomain features with a stable combination of vortex states and the violent motion of magnetic vectors contribute to the improvement of magnetic storage capacity and loss capability, which are evidenced by the off-axis electron holography and micromagnetic simulation. Consequently, the succulent-like FeCo exhibits enhanced permeability and microwave absorption performance. The effective absorption bandwidth reaches 5.68 GHz, and the maximum reflection loss is elevated to -53.81 dB. This work sheds considerable insight into the microstructure regulation with an application in microwave absorption and offers guidance in research for the topological magnetic configuration and dynamic response mechanism of magnetic alloys.

Customizing Heterointerfaces in Multilevel Hollow Architecture Constructed by Magnetic Spindle Arrays Using the Polymerizing-Etching Strategy for Boosting Microwave Absorption.

Heterointerface engineering is evolving as an effective approach to tune electromagnetic functional materials, but the mechanisms of heterointerfaces on microwave absorption (MA) remain unclear. In this work, abundant electromagnetic heterointerfaces are customized in multilevel hollow architecture via a one-step synergistic polymerizing-etching strategy. Fe/Fe3 O4 @C spindle-on-tube structures are transformed from FeOOH@polydopamine precursors by a controllable reduction process. The impressive electromagnetic heterostructures are realized on the Fe/Fe3 O4 @C hollow spindle arrays and induce strong interfacial polarization. The highly dispersive Fe/Fe3 O4 nanoparticles within spindles build multi-dimension magnetic networks, which enhance the interaction with incident microwaves and reinforce magnetic loss capacity. Moreover, the hierarchically hollow structure and electromagnetic synergistic components are conducive to the impedance matching between absorbing materials and air medium. Furthermore, the mechanisms of electromagnetic heterointerfaces on the MA are systematically investigated. Accordingly, the as-prepared hierarchical Fe/Fe3 O4 @C microtubes exhibit remarkable MA performance with a maximum refection loss of -55.4 dB and an absorption bandwidth of 4.2 GHz. Therefore, in this study, the authors not only demonstrate a synergistic strategy to design multilevel hollow architecture, but also provide a fundamental guide in heterointerface engineering of highly efficient electromagnetic functional materials.

Self-Assembly MXene-rGO/CoNi Film with Massive Continuous Heterointerfaces and Enhanced Magnetic Coupling for Superior Microwave Absorber.

MXene, as a rising star of two-dimensional (2D) materials, has been widely applied in fields of microwave absorption and electromagnetic shielding to cope with the arrival of the 5G era. However, challenges arise due to the excessively high permittivity and the difficulty of surface modification of few-layered MXenes severely, which infect the microwave absorption performance. Herein, for the first time, a carefully designed and optimized electrostatic self-assembly strategy to fabricate magnetized MXene-rGO/CoNi film was reported. Inside the synthesized composite film, rGO nanosheets decorated with highly dispersed CoNi nanoparticles are interclacted into MXene layers, which effectively suppresses the originally self-restacked of MXene nanosheets, resulting in a reduction of high permittivity. In addition, owing to the strong magnetic coupling between the magnetic FeCo alloy nanoparticles on the rGO substrate, the entire MXene-rGO/CoNi film exhibits a strong magnetic loss capability. Moreover, the local dielectric polarized fields exist at the continuous hetero-interfaces between 2D MXene and rGO further improve the capacity of microwave loss. Hence, the synthesized composite film exhibits excellent microwave absorption property with a maximum reflection loss value of - 54.1 dB at 13.28 GHz. The electromagnetic synergy strategy is expected to guide future exploration of high-efficiency MXene-based microwave absorption materials.

Atomic Short-Range Order in a Cation-Deficient Perovskite Anode for Fast-Charging and Long-Life Lithium-Ion Batteries.

Perovskite-type oxides are widely used for energy conversion and storage, but their rate-inhibiting phase transition and large volume change hinder the applications of most perovskite-type oxides for high-rate electrochemical energy storage. Here, it is shown that a cation-deficient perovskite CeNb3 O9 (CNO) can store a sufficient amount of lithium at a high charge/discharge rate, even when the sizes of the synthesized particles are on the order of micrometers. At 60 C (15 A g-1 ), corresponding to a 1 min charge, the CNO anode delivers over 52.8% of its capacity. In addition, the CNO anode material exhibits 96.6% capacity retention after 2000 charge-discharge cycles at 50 C (12.5 A g-1 ), indicating exceptional long-term cycling stability at high rates. The excellent cycling performance is attributed to the formation of atomic short-range order, which significantly prevents local and long-range structural rearrangements, stabilizing the host structure and being responsible for the small volume evolution. Moreover, the extremely high rate capacity can be explained by the intrinsically large interstitial sites in multiple directions, intercalation pseudocapacitance, atomic short-range order, and cation-vacancy-enhanced 3D-conduction networks for lithium ions. These structural characteristics and mechanisms can be used to design advanced perovskite electrode materials for fast-charging and long-life lithium-ion batteries.

High-Density Anisotropy Magnetism Enhanced Microwave Absorption Performance in Ti3C2Tx MXene@Ni Microspheres.

Two-dimensional materials, especially the newly emerging MXene, have attracted numerous interests in the fields of energy conversion/storage and electromagnetic shielding/absorption. However, the inherently inevitable aggregation and absence of magnetic loss of MXene considerably limit its electromagnetic absorption application. The introduction of magnetic component and favorable structural engineering are the alternatives to improve the microwave absorption (MA) performance. Herein, we report a spheroidization strategy to assemble double-shell MXene@Ni microspheres, where the commonly lamellar MXene are reshaped into three-dimensional microspheres that provide the substrate for oriented growth of Ni nanospikes. Whereas this structural feature offers massive accessible active surfaces that effectively promote the dielectric loss ability, the introduction of magnetic Ni nanospikes enables the additional magnetic loss capacity. Benefiting from these merits, the synthesized 3D MXene@Ni microspheres exhibit superior MA performance with the minimum reflection loss value of -59.6 dB at an ultrathin thickness (∼1.5 mm) and effective absorption bandwidth of 4.48 GHz. Moreover, the electron holography results reveal that the high-density anisotropy magnetism plays an important role in the improvement of MA performance, which provides an insight for the design of MXene-based materials as high-efficient microwave absorbers.

Hierarchical Ti3 C2 Tx MXene/Carbon Nanotubes Hollow Microsphere with Confined Magnetic Nanospheres for Broadband Microwave Absorption.

Hierarchical hollow structure with unique interfacial properties holds great potential for microwave absorption (MA). Ti3 C2 Tx MXene has been a hot topic due to rich interface structure, abundant defects, and functional groups. However, its overhigh permittivity and poor aggregation-resistance limit the further application. Herein, a hierarchical MXene-based hollow microsphere is prepared via a facile spray drying strategy. Within the microsphere, few-layered MXene nanosheets are separated by dispersed carbon nanotubes (CNTs), exposing abundant dielectric polarization interfaces. Besides, numerous magnetic Fe3 O4 nanospheres are uniformly dispersed and confined within nano-cavities between 1D network and 2D framework. Such a novel structure simultaneously promotes interfacial polarization by ternary MXene/CNTs/Fe3 O4 interfaces, enhances magnetic loss by microscale and nanoscale coupling network, enlarges conduction loss by MXene/CNTs dual-network, and optimizes impedance matching by hierarchical porous structure. Therefore, Fe3 O4 @Ti3 C2 Tx /CNTs composite achieves excellent MA property with a maximum reflection loss of -40.1 dB and an effective bandwidth of 5.8 GHz at the thickness of only 2 mm. This work demonstrates a feasible hierarchical structure design strategy for multi-dimension MXene composite to realize the high-efficiency MA performance.

Synthesis of Nonspherical Hollow Architecture with Magnetic Fe Core and Ni Decorated Tadpole-Like Shell as Ultrabroad Bandwidth Microwave Absorbers.

The advancement of electromagnetic (EM) protection technology promotes the urgent demand for the structural design of electromagnetic functional materials. Here, tadpole-like Fe@SiO2 @C-Ni (FSCN) composites with magnetic core-shell and nonspherical hollow architectures through multiple hydrolysis-polymerization reactions are reported. The Fe core and well-distributed Ni nanoparticles greatly promote the magnetic properties of FSCN and construct a multiscale magnetic coupling network. Meanwhile, the multishell composites consisting of carbon shell with Ni decorated possess an abundance of heterogeneous interfaces, generating effective interfacial polarization and relaxation. The hollow feature and the coordination of magnetic and dielectric capacities offer an optimized impedance balance, providing a fundament for the microwaves propagating into the absorber. Owing to the attractive effects resulted from the deliberate tadpole-like structure design, the FSCN composites ensure an effective EM energy conversion at the high-frequency region, which obtain the strongest reflection loss value of -45.2 dB and the extremely broad effective absorption bandwidth of 13.1 GHz. This work provides an important solution for magnetic-dielectric nanostructure design for microwave absorption and energy conversion materials.

3D Seed-Germination-Like MXene with In Situ Growing CNTs/Ni Heterojunction for Enhanced Microwave Absorption via Polarization and Magnetization.

HIGHLIGHTS: Benefiting from the possible "seed-germination" effect, the "seeds" Ni2+ grow into "buds" Ni nanoparticles and "stem" carbon nanotubes (CNTs) from the enlarged "soil" of MXene skeleton. Compared with the traditional magnetic agglomeration, the MXene-CNTs/Ni hybrids exhibit the highly spatial dispersed magnetic architecture. 3D MXene-CNTs/Ni composites hold excellent microwave absorption performance (-56.4 dB at only 2.4 mm). Ti3C2Tx MXene is widely regarded as a potential microwave absorber due to its dielectric multi-layered structure. However, missing magnetic loss capability of pure MXene leads to the unmatched electromagnetic parameters and unsatisfied impedance matching condition. Herein, with the inspiration from dielectric-magnetic synergy, this obstruction is solved by fabricating magnetic CNTs/Ni hetero-structure decorated MXene substrate via a facile in situ induced growth method. Ni2+ ions are successfully attached on the surface and interlamination of each MXene unit by intensive electrostatic adsorption. Benefiting from the possible "seed-germination" effect, the "seeds" Ni2+ grow into "buds" Ni nanoparticles and "stem" carbon nanotubes (CNTs) from the enlarged "soil" of MXene skeleton. Due to the improved impedance matching condition, the MXene-CNTs/Ni hybrid holds a superior microwave absorption performance of - 56.4 dB at only 2.4 mm thickness. Such a distinctive 3D architecture endows the hybrids: (i) a large-scale 3D magnetic coupling network in each dielectric unit that leading to the enhanced magnetic loss capability, (ii) a massive multi-heterojunction interface structure that resulting in the reinforced polarization loss capability, confirmed by the off-axis electron holography. These outstanding results provide novel ideas for developing magnetic MXene-based absorbers.

Hierarchical Magnetic Network Constructed by CoFe Nanoparticles Suspended Within "Tubes on Rods" Matrix Toward Enhanced Microwave Absorption.

Hierarchical magnetic-dielectric composites are promising functional materials with prospective applications in microwave absorption (MA) field. Herein, a three-dimension hierarchical "nanotubes on microrods," core-shell magnetic metal-carbon composite is rationally constructed for the first time via a fast metal-organic frameworks-based ligand exchange strategy followed by a carbonization treatment with melamine. Abundant magnetic CoFe nanoparticles are embedded within one-dimensional graphitized carbon/carbon nanotubes supported on micro-scale Mo2N rod (Mo2N@CoFe@C/CNT), constructing a special multi-dimension hierarchical MA material. Ligand exchange reaction is found to determine the formation of hierarchical magnetic-dielectric composite, which is assembled by dielectric Mo2N as core and spatially dispersed CoFe nanoparticles within C/CNTs as shell. Mo2N@CoFe@C/CNT composites exhibit superior MA performance with maximum reflection loss of - 53.5 dB at 2 mm thickness and show a broad effective absorption bandwidth of 5.0 GHz. The Mo2N@CoFe@C/CNT composites hold the following advantages: (1) hierarchical core-shell structure offers plentiful of heterojunction interfaces and triggers interfacial polarization, (2) unique electronic migration/hop paths in the graphitized C/CNTs and Mo2N rod facilitate conductive loss, (3) highly dispersed magnetic CoFe nanoparticles within "tubes on rods" matrix build multi-scale magnetic coupling network and reinforce magnetic response capability, confirmed by the off-axis electron holography.

Single Zinc Atoms Anchored on MOF-Derived N-Doped Carbon Shell Cooperated with Magnetic Core as an Ultrawideband Microwave Absorber.

Polarization behaviors of no-magnetic shell dominate the dielectric properties for core-shell magnetic-carbon composites, which faces a huge challenge. Herein, a single atom-doping strategy is established to adjust local electric potential in the metal-organic framework (MOF)-derived carbon shell. Benefiting from the confined transformation, single Zn atoms and N atoms are evenly distributed in the porous carbon shell using ZIF-8 as a template. Dielectric assembled carbon layers with functionalized Fe3 O4 core construct unique magnetic-dielectric synergy system. The electromagnetic parameters of Fe3 O4 @Zn-N-Carbon composites can be modified by tuning the pod-like Zn-N-doping carbon shell via repeating ZIF-8 growth cycles. Surprisingly, the core-shell Fe3 O4 @Zn-N-Carbon exhibits superior microwave absorption (MA) performance both in the reflection loss ability and wide-frequency responding feature. The reflection loss value of Fe3 O4 @Zn-N-Carbon microspheres reach -61.9 dB and the effective absorption bandwidth up to 11.5 GHz at only 2.5 mm thickness. The excellent MA mechanism is ascribed to following reasons. High-density stacking Zn-N doping carbon layers boost the interfacial polarization and plentiful Zn single atoms maximize the dipole polarization because of maximum atom utilization efficiency. Enhanced magnetic loss ability results from the compulsory magnetic coupling responding among Fe3 O4 cores. Magnetic-dielectric synergy of core-shell Fe3 O4 @Zn-N-Carbon microspheres can build ultrawide MA frequency.

C/MnO@void@C with Triple Balances for Superior Microwave Absorption Performance.

It is very promising and challenging to construct a yolk-shell structure with highly efficient microwave absorption (MA) performance through a simple fabrication process. Here, a novel C/MnO@void@C (MCC) yolk-shell structure has been successfully synthesized by one-step calcination without additional processing. The as-obtained MCC composites with tunable crystallinity degrees and hollowness can be obtained by treatment at various temperatures. The MCC composites treated at 700 °C (MCC-700) show an impressive MA performance, and the optimal reflection loss of -53.2 dB and an effective absorption bandwidth of 5.4 GHz can be obtained. This excellent performance results from multiple balance mechanisms. First, the regulated permittivity of MCC-700 due to proper crystallinity and hollowness is beneficial for the balance between dielectric loss (tan δε) and impedance match (Zim). Second, the optimal balance between the increasing polarization range and decreasing polarization intensity can be achieved, which is favorable for the improvement of the MA performance. Third, the multicore yolk-shell structure of MCC-700 is conducive to multiple scattering and continuous energy dissipation. Thus, our new findings provide a rational way for the utilization of yolk-shell structural manganese-based materials.

Confined Magnetic-Dielectric Balance Boosted Electromagnetic Wave Absorption.

Magnetic-dielectric property plays a critical significance for the functional expression toward advanced materials. Within nanoscale, the simultaneous regulation of the electrical and magnetic properties of electromagnetic (EM) wave absorption materials faces huge challenges. Herein, using the metal-organic frameworks (MOF) as templates, highly-dispersed ZnO and Co nanoparticles are uniformly confined inside graphited N-doped carbon skeleton, constructing the balanced EM property in the Co@NC-ZnO absorbers. Meanwhile, a dynamics and symmetrical morphology optimization of MOF-derived Co@NC-ZnO are dependent on the Co/Zn mass ratio and adjusting MOF frameworks, which evolves from the cube, truncated cube, dodecahedron, and to the final microsphere. Simultaneously, both the electronic conduction network and magnetic coupling network are compatible together in the in situ transformed Co@NC-ZnO system. Boosted magnetic responding ability and unique magnetic coupling are verified by the off-axis electronic holography. Plentiful heterojunction interfaces and special electronic conduction paths can be built in this Co-Zn-MOF derivatives, facilitating the dielectric loss behaviors. As expected, MOF-derived Co@NC-ZnO absorber displays outstanding EM wave absorption ability with strongest reflection loss value of -69.6 dB at only 1.9 mm thickness and wideband absorption covering 6.8 GHz at 2.4 mm. Confined EM balance provides new design strategy toward MOF-derived excellent MA materials and functional devices.

Compressible and flexible PPy@MoS2/C microwave absorption foam with strong dielectric polarization from 2D semiconductor intermediate sandwich structure.

Structural engineering represents a major trend in the field of two-dimensional (2D) materials regarding microscopic interfacial electric/dielectric properties and macroscopic device strategies. 2D molybdenum disulfide (MoS2) with semiconductive features and lamellar architecture has been widely applied in the microwave absorption (MA) field. However, due to its limitations of weak dielectric loss capacity and poor intrinsic mechanical property, MoS2-based MA devices are a considerable design challenge for practical applications with the peculiarities of light weight, high absorption performance, flexibility, and compressibility. Herein, 2D MoS2 was riveted on carbonized melamine foam (CMF) templated from a commercial foam skeleton, which was cladded with the conductive polymer polypyrrole (PPy). The as-prepared PPy@MoS2/CMF was integrated to simultaneously achieve an excellent MA performance including a maximum reflection loss (RL) value of -45.40 dB and a wide absorption bandwidth of 3.8 GHz, together with mechanical practicability including a high compression ratio of over 45.6% in volume and a bending angle of over 43.2°. This excellent MA performance is attributed to the synergetic effect from its sandwiched multi-layered skeleton, consisting of a conductive/semiconductive/conductive ternary conductive network, and multiple polarizations from the 2D MoS2 interlayer. Our strategy sheds novel insight into the construction of advanced carbon-supported composites and 2D materials for use in devices, which can be further extended to energy storage and conversion applications.

Magnetized MXene Microspheres with Multiscale Magnetic Coupling and Enhanced Polarized Interfaces for Distinct Microwave Absorption via a Spray-Drying Method.

As a typical 2D (two dimensional) material, Ti3C2Tx, has been used as a promising microwave absorber (MA) because of its massive interface architecture, abundant natural defects, and chemical surface functional groups. However, its single dielectric-type loss and excessive high conductivity seriously restrict the further enhancement of MA performance. Herein, we first describe a simple spray-drying routine to reshape the 2D MXene into a confined and magnetized microsphere with tightly embedded Fe3O4 nanospheres (designated as M/F), contributing to the enhanced specific interfaces and strong dielectric polarization. These Fe3O4 magnetic units are highly dispersed into the dielectric Mxene framework, leading to the optimized impedance balance and electromagnetic coordination capability. This composite way effectively exceeds the conventionally physical mixing, simple loading, and local phase separation method. Meanwhile, strong magnetic loss capability with significantly improved magnetic flux line density is achieved from microscale MXene and nanoscale Fe3O4, confirming our 3D multiscale magnetic coupling network. Accordingly, the M/F composites hold distinct microwave absorption property with the strong reflection loss (-50.6 dB) and effective absorption bandwidth (4.67 GHz) at the thickness as thin as only 2 mm. Our encouraging strategy provides important designable implications for MXene-based functional materials and high-performance absorbers.

Engineering Phase Transformation of MoS2/RGO by N-doping as an Excellent Microwave Absorber.

As a hot two-dimensional (2D) material, molybdenum disulfide has been attracting extensive attention for electromagnetic wave response applications because of its unique structure. However, the electronic conductivity of nanostructured MoS2 needs to be optimized urgently. Here, nitrogen-doped 1T@2H-MoS2/reduced graphene oxide (RGO) composites are effectively constructed by hydrothermal reaction and consecutive calcination under an NH3 atmosphere. The prepared composites possess great microwave absorption (MA) performance with an expected absorption bandwidth (4.00 GHz) at the Ku band and a maximum reflection loss value (-67.77 dB), which is much better than the performance of conventional 2H-MoS2 or 2H-MoS2/RGO. The prominent absorption property is ascribed to the (i) unique self-assemble morphology of rose-like MoS2 supported on 2D RGO; (ii) controllable crystalline phase switch between 2H and 1T; and (iii) brilliant energy attenuation caused by the intense multipolarization. Furthermore, the dominant MA mechanism is described as the local polarization motivated by the interaction between RGO and MoS2. Thus, our novel structure design provides a necessary reference to achieve optimized absorption performance based on 2D materials.

Multidimension-Controllable Synthesis of MOF-Derived Co@N-Doped Carbon Composite with Magnetic-Dielectric Synergy toward Strong Microwave Absorption.

Metal-organic framework (MOF) is highly desirable as a functional material owing to its low density, tunable pore size, and diversity of coordination formation, but limited by the poor dielectric properties. Herein, by controlling the solvent and mole ratio of cobalt/linker, multidimension-controllable MOF-derived nitrogen-doped carbon materials exhibit tunable morphology from sheet-, flower-, cube-, dodecahedron- to octahedron-like. Tunable electromagnetic parameters of Co@N-doped carbon composites (Co@NC) can be obtained and the initial MOF precursor determines the distribution of carbon framework and magnetic cobalt nanoparticles. Carbonized Co@NC composites possess the following advantages: i) controllable dimension and morphology to balance the electromagnetic properties with evenly charged density distribution; ii) magnetic-carbon composites offer plenty of interfacial polarization and strong magnetic coupling network; iii) a MOF-derived dielectric carbon skeleton provides electronic transportation paths and enhances conductive dissipation. Surface-mediated magnetic coupling reflects the stray magnetic flux field, which is corroborated by the off-axis electron holography and micro-magnetic simulation. Optimized octadecahedral Co@NC sample exhibits the best microwave absorption (MA) of -53.0 dB at the thickness of 1.8 mm and broad effective frequency from 11.4 to 17.6 GHz (Ku-band). These results pave the way to fabricate high-performance MA materials with balanced electromagnetic distribution and controlled morphology.

MOF-Derived Ni1-xCox@Carbon with Tunable Nano-Microstructure as Lightweight and Highly Efficient Electromagnetic Wave Absorber.

Intrinsic electric-magnetic property and special nano-micro architecture of functional materials have a significant effect on its electromagnetic wave energy conversion, especially in the microwave absorption (MA) field. Herein, porous Ni1-xCox@Carbon composites derived from metal-organic framework (MOF) were successfully synthesized via solvothermal reaction and subsequent annealing treatments. Benefiting from the coordination, carbonized bimetallic Ni-Co-MOF maintained its initial skeleton and transformed into magnetic-carbon composites with tunable nano-micro structure. During the thermal decomposition, generated magnetic particles/clusters acted as a catalyst to promote the carbon sp2 arrangement, forming special core-shell architecture. Therefore, pure Ni@C microspheres displayed strong MA behaviors than other Ni1-xCox@Carbon composites. Surprisingly, magnetic-dielectric Ni@C composites possessed the strongest reflection loss value - 59.5 dB and the effective absorption frequency covered as wide as 4.7 GHz. Meanwhile, the MA capacity also can be boosted by adjusting the absorber content from 25% to 40%. Magnetic-dielectric synergy effect of MOF-derived Ni1-xCox@Carbon microspheres was confirmed by the off-axis electron holography technology making a thorough inquiry in the MA mechanism.